For liquid crystal display devices, the classification based on the operating mode for liquid crystal molecules includes modes such as phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), fringe field switching (FFS), vertical alignment (VA) and polymer sustained alignment (PSA). The classification based on the driving mode of the device includes passive matrix (PM) and active matrix (AM) types. The PM type is further classified into static type, multiplex type and so on, and the AM type classified into thin film transistor (TFT) type, metal-insulator-metal (MIM) type and so on. The TFT type is further classified into amorphous silicon and polysilicon types, wherein the latter is classified into high-temperature type and low-temperature type according to the production process. The classification based on the light source includes the reflection type utilizing natural light, the transmission type utilizing a backlight, and the semi-transmission type utilizing both kinds of light.
These devices contain a liquid crystal composition having suitable characteristics. The liquid crystal composition has a nematic phase. The general characteristics of the composition should be improved to give an AM device having good general characteristics. Table 1 below summarizes the relationship between these two groups of general characteristics. The general characteristics of the composition will be further explained on the basis of a commercially available AM device. The temperature range of a nematic phase relates to the temperature range in which the device can be used. A desirable maximum temperature of the nematic phase is approximately 70° C. or higher and a desirable minimum temperature of the nematic phase approximately is −10° C. or lower. The viscosity of the composition relates to the response time of the device. A short response time is desirable for displaying moving images on the device. Accordingly, a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
TABLE 1General Characteristics of Composition and AM DeviceGeneral CharacteristicsGeneral CharacteristicsNo.of Compositionof AM Device1wide temperature range ofwide temperature range innematic phasewhich device can be used2small viscosity1)short response time3Suitable optical anisotropylarge contrast ratio4positively or negatively largelow threshold voltage anddielectric anisotropysmall electric power consumptionlarge contrast ratio5large specific resistancelarge voltage holding ratio andlarge contrast ratio6high stability to ultravioletlong service lifelight and heat1)A liquid crystal composition can he injected into a liquid crystal display device in a shorter period of time
The optical anisotropy of the composition relates to the contrast ratio of the device. The product (Δn×d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on the kind of operating mode. In a device having a VA mode or a PSA mode, a suitable value is in the range of approximately 0.30 micrometer to approximately 0.40 micrometer, and in a device having an IPS mode, a suitable value is in the range of approximately 0.20 micrometer to approximately 0.30 micrometer. In these cases, a composition having a large optical anisotropy is desirable for a device having a small cell gap. A large absolute value of the dielectric anisotropy in the composition contributes to a low threshold voltage, small electric power consumption and a large contrast ratio of the device. Accordingly, a large absolute value of the dielectric anisotropy is desirable. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. Accordingly, it is desirable that a composition should have a large specific resistance at room temperature and also at a high temperature in the initial stage. It is desirable that a composition should have a large specific resistance at room temperature and also at a high temperature after it has been used for a long time. The stability of the composition to ultraviolet light and heat relates to the service life of the liquid crystal display device. In the case where the stability is high, the device has a long service life. Such characteristics are desirable for an AM device used in a liquid crystal projector, a liquid crystal television and so forth.
A composition having positive dielectric anisotropy is used for an AM device having a TN mode. On the other hand, a composition having negative dielectric anisotropy is used for an AM device having a VA mode. A composition having positive or negative dielectric anisotropy is used for an AM device having an IPS mode or an FFS mode. A composition having positive or negative dielectric anisotropy is used for an AM device having a PSA mode. Regarding the AM device having a PSA mode, examples of a liquid crystal composition having negative dielectric anisotropy are disclosed in Patent Documents No. 1 to 6.